OBS compeses, scales and renders your scene via GPU, so a hit on GPU performance is normal (with 1440p scaled down to 720p 60fps my GTX1070 @3D clocks has ~6-10% load from OBS).
It's possible, that you have some browser sources or video device sources, that will increase that performance hit.
So maybe you can create a new scene collection and only add the game capture source to check, if the performance hit is the same, or much butter.
Btw. this rendering process is happening, even with OBS just opened and not recording/streaming.